PHYSICOCHEMICAL AND BIOLOGICAL STRESSORS 125 



lack of topographic relief, and geologic composition (permeable 

 limestones) prevent continuing existence of hyposaline biotopes (for 

 more detailed descriptions, see Norse, 1975, and Norse and Estevez, 

 1977). 



RESULTS AND DISCUSSION 



Since 1969, 1 have examined and measured the Callinectes 

 collections in the Allan Hancock Foundation of the University of 

 Southern California, the Harvard Museum of Comparative Zoology, 

 and the American Museum of Natural History and have collected 

 and/or observed in the field about 8000 portunids belonging to 26 

 tropical American species (Table 1). Tv^o species are not primarily 

 demersal; Euphylax dovii Stimpson is an epipelagic species that 

 enters inshore v^aters to spaw^n (Norse and Fox-Norse, 1977), and 

 Portunus sayi (Gibbes) lives in floating Sargassum patches that 

 occasionally drift ashore (Williams, 1965). Several species w^ere too 

 rarely found or locally distributed to permit any but tentative 

 conclusions about their distributions. Some species are commonly 

 associated w^ith particular plant communities or substrate types, but, 

 except to note that most occur mainly on soft bottoms, 1 will 

 concentrate on distributions w^ith regard to cUmatic and biological 

 stressors. 



From upriver to continental-shelf Pacific Colombian waters, 

 surface temperatures increased steadily from 26.3 to 28.4°C. This 

 small change would not be likely to cause great changes in species 

 compositions of demersal crabs, but rather indicates the change in 

 the ratio of cooler freshwater to warmer seawater. The surface 

 conditions, which directly affect benthos only in the shallows, reflect 

 deeper bottom conditions, depending on the hydrographic character- 

 istics of the estuaries (Gibbs, 1970). The most obvious major climatic 

 difference between river water and shelf water is salinity. 



Distributions of portunids and other groups reflect this salinity 

 gradient. Portunids comprised nearly 99% of the total of benthic 

 brachyuran crabs. Table 2 gives the species compositions (mean 

 relative abundances) of demersal portunids along the transects. 

 Callinectes toxotes Ordway dominates the freshest sites but is 

 replaced by C. arcuatus Ordway, which is then replaced by Euphylax 

 robustus A. Milne Edwards and Portunus asper (A. Milne Edwards) in 

 increasingly equable biotopes. 1 found a few C. arcuatus inshore in 

 Panama, but, in the northern Gulf of California, there is a clear 

 ecological separation between Callinectes spp. Tidal channels drain- 

 ing salt marshes and shallow areas of both bays and the open gulf are 



